Threshold Pion Electroproduction in Chiral Perturbation Theory

Electroproduction of pions on the nucleon near the threshold is analyzed within the framework of baryon chiral perturbation theory. We give a thorough discussion of the low--energy theorems related to charged and neutral electropionproduction. It is shown how the axial radius of the nucleon can be related to the S--wave multipoles $E_{0+}^{(-)}$ and $L_{0+}^{(-)}$. The chiral perturbation theory calculations of the $\gamma^\star p \to \pi^0 p$ reaction are found to be in good agreement with the recent near threshold data. We also discuss the influence of some isospin--breaking effects in this channel. For future experimental tests of the underlying chiral dynamics, extensive predictions of differential cross sections and multipole amplitudes are presented.